Effects of meal-feeding on lifespan of mice will be investigated as a function of the timing of a restricted span of food accessibility in relation to stages of the daily light-dark regimen. Any such effects of meal-timing will be related to effects on circadian rhythms in body temperature, hormones and related variables. Effects on body weight and body composition will also be determined. Food consumption will be monitored so that control groups can be pair-fed by amount but according to ad libitum feeding behavior. Effects of the meal-feeding pattern of food consumption (as contrasted with the ad libitum pattern) can thus be separated from effects of reduced food intake. Results from this investigation will be viewed in terms of: 1) benefits to be gained from a consideration of the timing of daily food consumption; 2) relations between circadian rhythm alteration and senescence.